The invention concerns a process and a device for detecting latent flatness defects of a metal band product running along a longitudinal direction.
When rolling metal bands, and notably thin rolled sheet, flatness defects may be observed once the band is in resting position, but that may be in latent condition in the band, as said band is maintained under tensile load. Such defects are due generally to small variations over the width of the band, to the elongation carried out by the rolling process and may be corrected while acting on the rolling conditions and, in particular, on the distribution, in the transversal direction, of the clamping pressure applied between the working rolls.
To that effect, modern rolling mills are fitted, usually, with means enabling to correct the gap profile to the let the product through
Often, the stand of the rolling mill is fitted with actuators bearing upon the chocks of the working rolls in order to bring said rolls closer or apart, with a cambering effect of the rolls.
In another arrangement, fractional spraying means enable to act thermally on the profile of the rolls.
It is thus possible to act on the profile of at least one back-up roll which comprises a ductile envelope mounted to rotate around a fixed shaft and bearing upon said shaft by a plurality of actuators whereof the position and the pressure are adjustable, whereas said actuators are distributed over the width of the band.
Usually, these means of adjustment are controlled thanks to information given by a measuring device placed downstream of the rolling mill and sensitive to the variations, over the width of the band, of the tensile load applied to said band, which correspond themselves to the variations in elongation of the longitudinal fibres of the band.
Such a measuring device consists, generally, of a deflector roll comprising a cylindrical body mounted to rotate around an axis perpendicular to the longitudinal running direction of the band. Said band is applied under tensile load to an angular sector of the external surface of the roll that is fitted with a series of sensors enabling to measure the variations in the local application pressure of the band. These detectors are spaced from one another and distributed over the whole length of the roll, whereas the band is divided into a series of longitudinal zones each corresponding to a detection zone whereof the width is set, whereon the measurement conducted by the sensor is integrated in order to assess the latent defect to be corrected in the corresponding longitudinal zone.
To avoid interferences between the measurements conducted on two adjacent measuring zones, the sensors are advantageously offset angularly from one zone to the next.
Generally speaking, a measuring roll comprises therefore a plurality of detection zones distributed over its whole length and each fitted with a sensor for transmitting a signal related to the application pressure of the corresponding section of the band, when this detection zone passes through the angular sector where the band is in contact with the roll.
As the band is under tensile load, the roll must sustain the deflection and comprises therefore, usually, a central tubular body of sufficient thickness to provide the necessary strength and fitted, on its external surface, with a plurality of recesses wherein the measuring sensors are located, whereas each recess is enclosed, outwardly, by a protection wall to avoid direct contact between the sensor and the running band.
In certain arrangements known, the pressure applied by the band in the detection zone is measured directly by means of a strength sensor, for example of the piezo-electric or quartz type, which is interposed between the bottom of the recess and the protection wall whereon the band is applied.
To obtain information directly representative of the pressure applied, the protection wall may consist of the external surface of a cap-shaped piece that encloses the recess outwardly but a slight clearance ought to be left between this cap and the lateral surface of the recess, a clearance that may be filled with dirty particles (DE-A-19747655).
In certain arrangements known, the protection cap consists of a wall imbedded in a countersink provided on the periphery of the recess. However, as the sensor is clamped between the bottom of the recess and the protection wall which rest on the bottom of the countersink, the pressure measurement is disturbed by deflection resistance of this wall.
To remedy these shortcomings, it has been suggested in the document U.S. Pat. No. 3,324,695, to cover the roll with a continuous envelope applied onto the external surface of the resistant body of the roll as a hoop, whereas each recess of a sensor is covered by a portion of this envelope forming a thin protection wall.
In such a case, as described, for example, in document U.S. Pat. No. 4,356,714, the pressure applied by the band is not measured directly, but the deformation of the thin wall resulting from this pressure, by means of a position sensor having two elements mounted to slide radially one inside the other and resting, in opposite directions respectively, on the bottom of the recess and on the internal surface of the thin wall, in the centre of said wall.
This thin wall stretched above the sensor and sensitive to the pressure applied by the product, behaves like a plate imbedded on its edges and the conventional formulae in the mechanics of materials enable therefore to assess the pressure applied onto the ductile wall from the deflection measured at the centre of the plate.
Advantageously, the position sensors may be of the  less than  less than LVDT  greater than  greater than  type, having a primary winding and two secondary windings mounted opposite one another, between which variable magnetic coupling is created by a mobile core linked with the displacement of the stem of the sensor.
The roll is associated with an automatic measuring system which enables, at each rotation, to send a measuring pulse to the primary winding of each sensor and to read the induced signal at the terminals of the secondary windings. An angular position encoder enables to conduct the measurement under load when each sensor passes through an application zone of the band. Another measurement is conducted in empty condition when the sensor is situated inside the angular application sector. The difference between the measurement under load and the measurement in empty condition provides the deflection at the centre of the plate, from which the pressure applied can be determined and, consequently, the tensile load in the corresponding longitudinal zone of the band.
The measurement in empty condition can be conducted in any angular position of the roll which does not correspond to the measurement under load for one of the sensors. Said sensors are therefore distributed judiciously on the surface of the roll in order to enable easy management of the measurements in empty condition and under load, during a rotation of the roll (U.S. Pat. No. 4,356,714).
Until now, such flatness measurement rolls had only been used in cold rolling facilities for which the temperature of the band remains moderate.
Indeed, if a pressure sensor clamped between the bottom of the recess and the closing cap of said recess is used, the expansions of the different parts, in case of rising temperature, may cause excessive friction and, consequently, faulty measurements.
This shortcoming does not exist in the systems where position sensors covered with a closing plate are used. However, this plate consists of a thin wall which takes on the temperature of the band immediately and is therefore subject to thermal deformation. Moreover, as the rotational speed is high, the centrifugal force may also induce a slight deformation.
The comparison of the value under load of the signal with a value in empty condition measured at each rotation, enables to reset the sensor and to measure the actual deflection of the plate, but to do so, the body of the roll should remain at substantially constant temperature and this is only valid if the temperature of the band is moderate.
However, even in cold rolling facilities, the reduction in thickness induces certain heating-up of the band whereof the temperature may be close to 200xc2x0 C.
On the other hand, the flatness defects resulting from variations in elongation over the width of the band also appear during hot rolling and it would be therefore interesting to measure, as of that moment, the risks of future defects in order to correct them.
However, even for a non-ferrous metal such as aluminium, the hot band is at high temperature, around a few hundred degrees. If, for measuring, pressure sensors are used, it should be contemplated to cool each sensor down, for example by circulating a cooling fluid such as water. But the realisation of a roll fitted with cooling circuits is very complex.
The position sensors do not have this shortcoming as they may be provided to operate even at high temperatures since they consist, simply, of two elements sliding one into another. However, in case of high temperature, the thermal deformations of the closing plate of each recess are relatively important.
It ensues that, until now, it had not seemed possible to use flatness measuring rolls of conventional type in the case of hot rolling.
It is an object of the invention to solve these problems thanks to a new process that enables to realise accurate measurements of the distribution of the loads on a band even if said band is at high temperature.
The invention is therefore particularly suited to hot rolling facilities for which, until now, it seemed impossible to use an flatness roll.
However, because of the advantages obtained and the simplicity of the means employed, the invention can also be applied to cold rolling facilities since it enables to improve the accuracy of the measurement and, consequently, of the correction of flatness.
Besides part, the arrangements according to the invention enable to assess a variation profile of the temperature of the band over the width of said band in order to take said profile into account for correcting flatness defects.
The invention applies therefore, generally speaking, to a process for detecting latent flatness defects in a band product running along a longitudinal direction and applied under tensile load to an angular sector of a measuring roll comprising a cylindrical body mounted to rotate around an axis perpendicular to the running direction and having a external surface along which are distributed a plurality of detection zones centred on transversal planes spaced from one another and each comprising a transmission sensor, at each rotation, of a signal having a value measured under load, corresponding to the pressure applied to the detection zone by a corresponding longitudinal zone of the band when the detection zone passes through the angular sector of application of the band, a process wherein, at each rotation, the value measured under load of the signal transmitted by each sensor is compared with a reference value, in order to assess, in each longitudinal zone of the band, information representative of the tensile load applied in this zone.
According to the invention, two values in empty condition of the signal transmitted by each sensor are measured, respectively before and after the corresponding detection zone has passed through the angular sector of application of the band and the reference value to which is compared, at each rotation, the value under load of the signal transmitted by each sensor is obtained by combining said two values in empty condition, respectively preceding and subsequent of the signal transmitted by the same sensor.
Preferably, both values in empty condition of the signal are measured, respectively, immediately before and immediately after the corresponding detection zone has passed through the application sector of the band.
Practically, the reference value can be equal to the arithmetic average of both values in empty condition, respectively before and after the measurement under load. However, it is possible to combine both values in a different fashion in empty condition while weighing said values in order to take into account the temperature evolution during the rotation of the roll.
The invention is particularly suited to the flatness measuring rolls wherein the deformation of the protection wall in the centre of each detection zone is measured by means of a position sensor having one element resting at the bottom of the recess and one element resting on the protection wall. However, the invention also exhibits advantages for the other types of rolls since it enables, generally speaking, to improve the measurement accuracy.
The invention enables to control the flatness of a hot-rolled sheet, which did not seem possible previously. In such a case, it is particularly advantageous to realise forced cooling of the external surface of the roll over a sector of said surface extending between the zones where both values in empty condition of the signal transmitted by each sensor are measured, in order to bring the temperature of the roll down to substantially constant level before the detection zone is brought back within the application sector.
However, the invention may also be applied advantageously to cold rolling since it enables to improve the accuracy of the flatness measurement.
But the invention also exhibits other advantages. In particular, on the basis of the variation profile of the subsequent values in empty condition measured after each sensor has passed through the application sector, the temperature variation profile is assessed over the width of the band in order to deduce the influence of the thermal dilatation of each longitudinal zone on the flatness measurement performed in this zone, so that the flatness corrections assessed, for each zone, on the basis of said flatness measurement, correspond to an average temperature valid over the whole width of the band.
Thus, the flatness corrections can be determined in order to perform on the different longitudinal zones of the band in order to obtain the requested flatness profile on a sheet with homogeneous temperature and even after cooling at room temperature.